Chang-Hee Cho

Quantum confinement effect in crystalline silicon quantum dots in silicon nitride grown using SiH4 and NH3

Crystalline silicon quantum dots (Si QDs) were spontaneously grown in the silicon nitride films by plasma-enhanced chemical vapor deposition using SiH4 and NH3 as precursors. When the size of the Si QDs was reduced from 4.9 to 2.9nm, the photoluminescence peak energy was shifted from 1.73 to 2.77eV. The photoluminescence peak energy was fitted to the relationship, E(eV)=1.13+13.9∕d2, where d is the diameter of the Si QD in nanometers. The measured band-gap energies of the Si QDs were in good agreement with the quantum confinement model for crystalline Si QDs. These results suggest that the hydrogen dissociated from NH3 plays an important role in improving the crystallinity and surface passivation of Si QDs.

All-optical active switching in individual semiconductor nanowires

The imminent limitations of electronic integrated circuits are stimulating intense activity in the area of nanophotonics for the development of on-chip optical components, and solutions incorporating direct-bandgap semiconductors are important in achieving this end. Optical processing of data at the nanometre scale is promising for circumventing these limitations, but requires the development of a toolbox of components including emitters, detectors, modulators, waveguides and switches. In comparison to components fabricated using top-down methods, semiconductor nanowires offer superior surface properties and stronger optical confinement. They are therefore ideal candidates for nanoscale optical network components, as well as model systems for understanding optical confinement. Here, we demonstrate all-optical switching in individual CdS nanowire cavities with subwavelength dimensions through stimulated polariton scattering, as well as a functional NAND gate built from multiple switches. The device design exploits the strong light-matter coupling present in these nanowires, leading to footprints that are a fraction of those of comparable silicon-based dielectric contrast and photonic crystal devices.

Photoluminescence of silicon quantum dots in silicon nitride grown by NH3 and SiH4

The photoluminescence (PL) property of crystalline silicon quantum dots (Si QDs) in silicon nitride grown by using ammonia and silane gases is reported. The peak position of PL could be controlled in the wavelength range from 450 to 700 nm by adjusting the flow rates of ammonia and silane gases. The PL intensity of Si QDs grown by ammonia was more intense compared to that of Si QDs grown by nitrogen gas. To investigate the role of hydrogen in the PL enhancement, the Si QDs grown by nitrogen gas were postannealed under hydrogen ambient. The enhancement in PL intensity was attributed to the hydrogen passivation of dangling bonds related to silicon and/or nitrogen at the interface of Si QDs and silicon nitride.

Tailoring hot-exciton emission and lifetimes in semiconducting nanowires via whispering-gallery nanocavity plasmons

The manipulation of radiative properties of light emitters coupled with surface plasmons is important for engineering new nanoscale optoelectronic devices, including lasers, detectors and single photon emitters. However, so far the radiative rates of excited states in semiconductors and molecular systems have been enhanced only moderately, typically by a factor of 10-50, producing emission mostly from thermalized excitons. Here, we show the generation of dominant hot-exciton emission, that is, luminescence from non-thermalized excitons that are enhanced by the highly concentrated electromagnetic fields supported by the resonant whispering-gallery plasmonic nanocavities of CdS-SiO(2)-Ag core-shell nanowire devices. By tuning the plasmonic cavity size to match the whispering-gallery resonances, an almost complete transition from thermalized exciton to hot-exciton emission can be achieved, which reflects exceptionally high radiative rate enhancement of >10(3) and sub-picosecond lifetimes. Core-shell plasmonic nanowires are an ideal test bed for studying and controlling strong plasmon-exciton interaction at the nanoscale and opens new avenues for applications in ultrafast nanophotonic devices.

Silicon coupled with plasmon nanocavities generates bright visible hot luminescence

Due to limitations in device speed and performance of silicon-based electronics, silicon optoelectronics has been extensively studied to achieve ultrafast optical-data processing1–3. However, the biggest challenge has been to develop an efficient silicon-based light source since indirect band-gap of silicon gives rise to extremely low emission efficiency. Although light emission in quantum-confined silicon at sub-10 nm lengthscales has been demonstrated4–7, there are difficulties in integrating quantum structures with conventional electronics8,9. It is desirable to develop new concepts to obtain emission from silicon at lengthscales compatible with current electronic devices (20-100 nm), which therefore do not utilize quantum-confinement effects. Here, we demonstrate an entirely new method to achieve bright visible light emission in “bulk-sized” silicon coupled with plasmon nanocavities from non-thermalized carrier recombination. Highly enhanced emission quantum efficiency (>1%) in plasmonic silicon, along with its size compatibility with present silicon electronics, provides new avenues for developing monolithically integrated light-sources on conventional microchips.

One-dimensional polaritons with size-tunable and enhanced coupling strengths in semiconductor nanowires

Strong coupling of light with excitons in direct bandgap semiconductors leads to the formation of composite photonic-electronic quasi-particles (polaritons), in which energy oscillates coherently between the photonic and excitonic states with the vacuum Rabi frequency. The light-matter coherence is maintained until the oscillator dephases or the photon escapes. Exciton-polariton formation has enabled the observation of Bose-Einstein condensation in the solid-state, low-threshold polariton lasing and is also useful for terahertz and slow-light applications. However, maintaining coherence for higher carrier concentration and temperature applications still requires increased coupling strengths. Here, we report on size-tunable, exceptionally high exciton-polariton coupling strengths characterized by a vacuum Rabi splitting of up to 200 meV as well as a reduction in group velocity, in surface-passivated, self-assembled semiconductor nanowire cavities. These experiments represent systematic investigations on light-matter coupling in one-dimensional optical nanocavities, demonstrating the ability to engineer light-matter coupling strengths at the nanoscale, even in non-quantum-confined systems, to values much higher than in bulk.

Effect of hydrogen passivation on charge storage in silicon quantum dots embedded in silicon nitride film

The effect of hydrogen passivation on the charge storage characteristics of two types of silicon nitride films containing silicon quantum dots (Si QDs) grown by SiH4+N2 and SiH4+NH3 plasma was investigated. The transmission electron microscope analysis and the capacitance-voltage measurement showed that the silicon nitride film grown by SiH4+NH3 plasma has a lower interface trap density and a higher density of Si QDs compared to that grown by SiH4+N2 plasma. It was also found that the charge retention characteristics in the Si QDs were greatly enhanced in the samples grown by means of SiH4+NH3 plasma, due to the hydrogen passivation of the defects in the silicon nitride films by NH3 during the growth of the Si QDs.

Ni∕Au contact to silicon quantum dot light-emitting diodes for the enhancement of carrier injection and light extraction efficiency

The effect of Ni∕Au metal contact on the carrier injection and the electroluminescence of silicon quantum dot light-emitting diodes (LEDs) was investigated. An LED with an annealed Ni∕Au contact at 400°C in air showed a lower threshold voltage compared to that of an as-deposited Ni∕Au contact by forming a nickel silicide, which has a lower work function than Ni at the interface between metal layers and silicon nitride. The optical output power of the LED with the annealed Ni∕Au contact was also increased due to a highly transparent NiO layer and a lowly resistant Au layer.

Effect of InGaN quantum dot size on the recombination process in light-emitting diodes

The effect of InGaN quantum dot (QD) size on the performance of light-emitting diodes (LEDs) was investigated by varying the QD size from 1.32to2.81nm. The electroluminescence peak of the LEDs containing small QDs (1.32nm) was redshifted with increasing input current while that of large QDs (2.81nm) was blueshifted up to 40mA due to the screening effect of the piezoelectric field. The optical output power of LEDs fabricated with small QDs was much higher compared to those with large QDs. These results were attributed to a weaker piezoelectric field and enhanced quantum confinement in small QDs.

Tailoring light–matter coupling in semiconductor and hybrid-plasmonic nanowires

Understanding interactions between light and matter is central to many fields, providing invaluable insights into the nature of matter. In its own right, a greater understanding of light-matter coupling has allowed for the creation of tailored applications, resulting in a variety of devices such as lasers, switches, sensors, modulators, and detectors. Reduction of optical mode volume is crucial to enhancing light-matter coupling strength, and among solid-state systems, self-assembled semiconductor and hybrid-plasmonic nanowires are amenable to creation of highly-confined optical modes. Following development of unique spectroscopic techniques designed for the nanowire morphology, carefully engineered semiconductor nanowire cavities have recently been tailored to enhance light-matter coupling strength in a manner previously seen in optical microcavities. Much smaller mode volumes in tailored hybrid-plasmonic nanowires have recently allowed for similar breakthroughs, resulting in sub-picosecond excited-state lifetimes and exceptionally high radiative rate enhancement. Here, we review literature on light-matter interactions in semiconductor and hybrid-plasmonic monolithic nanowire optical cavities to highlight recent progress made in tailoring light-matter coupling strengths. Beginning with a discussion of relevant concepts from optical physics, we will discuss how our knowledge of light-matter coupling has evolved with our ability to produce ever-shrinking optical mode volumes, shifting focus from bulk materials to optical microcavities, before moving on to recent results obtained from semiconducting nanowires.